User apparatus, base station, and communication method

ABSTRACT

A user apparatus communicating with a base station in a radio communication system including the base station and the user apparatus includes a first receiving unit to receive multiple first reference signals transmitted from the base station; a detection unit to detect a specific antenna port via which the plurality of first reference signals is received, or a specific directivity pattern, among a plurality of directivity patterns generated by a plurality of antenna ports, in which the plurality of first reference signals are received; a measuring unit to measure reception power of each of the first reference signals; and a transmission unit to group the reception power of each of the first reference signals into one of groups of the specific antenna port and the specific directivity pattern via which the first reference signals are received to transmit the groups of the reception power to the base station.

TECHNICAL FIELD

The present invention relates to a user apparatus, a base station, and acommunication method.

BACKGROUND ART

In LTE/LTE-Advanced, MIMO (multiple-input and multiple-output)technology that increases system capacity, cell edge user throughput,and the like is employed. In addition, a heterogeneous networktechnology that achieves high quality communication by reducingintercell interference while mixing different types of base stations(macrocells, small cells, etc.) has been studied.

Specifically, it is assumed that high frequency bands are used in smallcells in heterogeneous networks. Note that propagation loss increases inhigh frequency bands. Hence, in order to compensate for such apropagation loss increase, application of massive MIMO that performsbeam forming with a narrow beam width has been studied. In addition,massive MIMO has attracted attention as element technology in Fifthgeneration (5G) radio technology.

Massive MIMO is a large-scale MIMO with a large number (e.g., 100elements) of antennas installed on the base station, which enablesstrength of the electric field to concentrate in a narrow area to reduceinterference between users as a result.

RELATED ART DOCUMENTS Patent Document

-   [Patent Document 1] Japanese Unexamined Patent Publication No.    2013-219507

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

FIG. 1 is a diagram illustrating an example of a communicationenvironment that includes a large number of small cells to which massiveMIMO is applied. As illustrated in FIG. 1, a large number of beams aretransmitted from a base station of each small cell. In a case where auser apparatus (UE) communicates while moving in such an environment, auser apparatus selects a specific beam adapted to a current position andperforms communication while sequentially switching the beams accordingto its movement. In addition, it is possible to perform MIMOcommunication by simultaneously using multiple beams formed by massiveMIMO.

In order to achieve such an operation, it is necessary for a userapparatus and a base station to determine multiple beams as candidatesfor communication by MIMO.

FIG. 2 is a diagram illustrating a problem to be solved by theinvention. In an example of FIG. 2(a), a user apparatus receives twobeams transmitted from a base station having a large number of antennaelements from the same direction. By contrast, in an example of FIG.2(b), a user apparatus receives two beams transmitted from a basestation from different directions.

Note that in order to improve quality of communication by MIMO, it isdesirable to have a lower correlation between channels constitutingMIMO. That is, in a case where the user apparatus simultaneously usesmultiple beams to perform MIMO transmission, it is desirable to performMIMO transmission using multiple beams received from differentdirections as illustrated in FIG. 2(b) compared to performing MIMOtransmission using multiple beams received from the same direction asillustrated in FIG. 2(a).

The disclosed technology has been developed in view of theabove-described point, and an object is to provide a technique for usein a radio communication system having a base station performing beamforming and a user apparatus, which enables appropriately selectingmultiple beams used for communication, from among multiple beams formedby the base station.

Means for Solving the Problem

A user apparatus according to the disclosed technique is a userapparatus communicating with a base station of a radio communicationsystem that includes the base station and the user apparatus, whereinthe user apparatus includes a first receiving unit configured to receivea plurality of first reference signals transmitted from the basestation; a detection unit configured to detect a specific antenna portvia which the plurality of first reference signals is received, or aspecific directivity pattern, among a plurality of directivity patternsgenerated by a plurality of antenna ports, in which the plurality offirst reference signals are received; a measuring unit configured tomeasure reception power of each of the plurality of first referencesignals; and a transmission unit configured to group the reception powerof each of the plurality of first reference signals into one of groupsof the specific antenna port and the specific directivity pattern viawhich the plurality of first reference signals are received to transmitthe groups of the reception power to the base station.

Effect of the Present Invention

The disclosed technology may provide a technology, in a radiocommunication system having a base station performing beam forming and auser apparatus, capable of appropriately selecting a plurality of beamsused for communication, among a plurality of beams formed by the basestation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a communicationenvironment that includes a large number of small cells to which massiveMIMO is applied;

FIG. 2 is a diagram illustrating a problem to be solved by theinvention;

FIG. 3 is a diagram illustrating an overall configuration of a radiocommunication system according to an embodiment;

FIG. 4 is a diagram illustrating a configuration of a beam used forcommunication and each reference signal;

FIG. 5 is a diagram illustrating a configuration of a beam used forcommunication;

FIG. 6 is a diagram illustrating an example of mapping of discoverysignals in an embodiment;

FIG. 7 is a diagram illustrating an example of a functionalconfiguration of a user apparatus according to an embodiment;

FIG. 8 is a diagram illustrating an example of a functionalconfiguration of a small base station according to an embodiment; and

FIG. 9 is a diagram illustrating an example of a hardware configurationof a user apparatus according to an embodiment;

FIG. 10 is a diagram illustrating an example of a hardware configurationof a small base station according to an embodiment;

FIG. 11 is a diagram illustrating a method for measuring a discoverysignal according to an embodiment;

FIG. 12 is a diagram illustrating the number of narrow beams of whichthe user apparatus reports a reception quality;

FIG. 13 is a diagram illustrating an example of a process sequence of amobile communication system according to an embodiment;

FIG. 14 is a diagram illustrating an example of a configuration of areception quality report signal;

FIG. 15 includes diagrams illustrating an example of format information;

FIG. 16 includes diagrams illustrating an example of a modification of aconfiguration of the reception quality report signal;

FIG. 17 includes diagrams illustrating an example of a modification ofthe format information;

FIG. 18 includes diagrams illustrating an example of a signal in a casewhere format information is reported from a base station;

FIG. 19 includes diagrams illustrating an example of a modification(part 1) of a signal in a case where format information is reported froma base station;

FIG. 20 includes diagrams illustrating an example of a modification(part 2) of a signal in a case where format information is reported froma base station;

FIG. 21 includes diagrams illustrating a method for transmitting areception quality report signal and feedback information;

FIG. 22 includes diagrams illustrating an example of format informationto which a report pattern is attached;

FIG. 23 includes diagrams illustrating a modification of formatinformation to which a report pattern is attached; and

FIG. 24 includes diagrams illustrating an example of a method fortransmitting a discovery signal corresponding to a narrow beam.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The following describes embodiments of the present invention withreference to the accompanying drawings. Note that the embodimentsdescribed below are merely examples and the embodiments to which thepresent invention is applied are not limited to the followingembodiments. For example, it is assumed that a radio communicationsystem according to an embodiment complies with LTE standards. However,the present invention may be applied not limited to LTE but also beapplied to other systems. Note that, in the specification and theclaims, the term “LTE” is used not only to mean a communication schemecorresponding to 3GPP release 8 or 9, but also to mean the communicationsystem corresponding to 3GPP release 10, 11, 12, 13, 14 or later.

Although a configuration of a reference signal of three hierarchies isbasically illustrated as an example in the present embodiment, thenumber of hierarchies is not limited to three, and the number ofhierarchies may be two, or the number of hierarchies may be four ormore.

<Outline>

(Overall Configuration of Radio Communication System)

FIG. 3 illustrates an overall configuration diagram of a radiocommunication system according to an embodiment. The radio communicationsystem according to the present embodiment includes a macro base station10 forming a macrocell and small base stations 11 and 12 in a coveragearea of the macrocell. In addition, FIG. 3 illustrates a user apparatus20 communicating with the macro base station 10, the small base stations11 and 12, and the like.

In the radio communication system, macro coverage is secured by themacro base station 10 with a low frequency band, and the small basestations 11 and 12 absorb traffic of a small area (e.g., hot spot) witha high frequency band; however, the allocation of such frequency bandsis merely an example, and the allocation of such frequency bands is thusnot limited to this example.

The macro base station 10, and the small base stations 11 and 12communicate with the user apparatus 20 via radio. Each of the macro basestation 10 and the small base stations 11 and 12 is configured toinclude hardware resources including a CPU such as one or moreprocessors, a memory device including a ROM, a RAM or a flash memory, anantenna for communicating with the user apparatus 20 or the like, and acommunication interface device for communicating with an adjacent basestation, a core network and the like. Respective functions and processesof the macro base station 10 and the small base stations 11 and 12 maybe implemented by causing the processors to process or executing dataand programs stored in the memory device. However, the macro basestation 10 and the small base stations 11 and 12 are not limited to theabove-described hardware configuration, and may have any otherappropriate hardware configuration.

The user apparatus 20 has a function to communicate with the macro basestation 10, the small base stations 11 and 12, the core network, and thelike through radio. The user apparatus 20 may, for example, be a mobilephone, a smartphone, a tablet, a mobile router, a wearable terminal, orthe like. The user apparatus 20 may be any user apparatus 20 insofar asthe user apparatus 20 has a communication function. The user apparatus20 is configured to include hardware resources including a CPU such asone or more processors, a memory device such as a ROM, a RAM, or a flashmemory, an antenna for communicating with the macro base station 10 andthe small base stations 11 and 12, and an RF (Radio Frequency) device.Respective functions and processes of the user apparatus 20 may beimplemented by causing the processors to process or execute the programsand data stored in the memory device. However, the user apparatus 20 isnot limited to the above-described hardware configuration; the userapparatus 20 may have any other appropriate hardware configuration.

The small base stations 11 and 12 in the present embodiment have thefunction of massive MIMO such that the small base stations 11 and 12 maybe enabled to form various beams from a wide beam to a narrow beam. Asillustrated in FIG. 3, according to the present embodiment, multiplepre-coded reference signals of different types are transmitted by beamsfrom each of the small base stations. Note that a reference signal being“pre-coded” indicates that, in a transmission example, a transmissionsignal is multiplied by the weight for each antenna port such that thereference signal is transmitted with a beam having a certain width. Forexample, in the example illustrated in FIG. 3, a reference signal istransmitted from the small base station 11 with beam 1-1, beam 1-2, andbeam 1-3, respectively. Further, a reference signal is transmitted fromthe small base station 12 with beam 2-1, beam 2-2, and beam 2-3,respectively.

In a coverage area of the macrocell, the user apparatus 20 receivesMacro-Assisted Information from the macro base station 10 (step S1), andthe user apparatus 20 monitors, by using macro auxiliary information, areference signal (this signal is referred to as a “Discovery Signal”)transmitted with beams of the small base stations 11 and 12 to receive(detect) a specific discovery signal.

In this case, it is assumed that the user apparatus 20 has received adiscovery signal transmitted with each beam of the small base station11. The user apparatus 20 measures reception quality (reception power,etc.) based on the received discovery signal and transmits a measuredresult of the reception quality to the small base station 11 (step S2).The small base station 11 determines, based on a measurement report (areport of the measured result), for example, the beam with which theuser apparatus 20 has successfully received a signal with the bestquality (e.g., the highest reception power), and transmits a referencesignal (hereinafter referred to as a “measurement reference signal”)with the determined beam. The user apparatus 20 measures the receptionquality based on the measurement reference signal and generates feedbackinformation (CSI (Channel State Information) or the like) to be reportedto the small base station 11, and transmits the generated CSI to thesmall base station 11.

Based on the feedback information received from the user apparatus 20,the small base station 11 allocates radio resources for transmittingdata signals (PDSCH and the like) to the user apparatus 20.

As described above, the user apparatus 20 may be enabled to receive thedata signal from the small base station 11 by sequentially performing astep of selecting candidate beams to be used for communication by usinga discovery signal, a step of determining a beam of the best quality,from the selected candidate beams, as a beam to be used forcommunication, and a step of allocating the radio resources based on thefeedback information with respect to the determined beam.

(Beam Identification Method)

Next, a description will be given, with reference to FIGS. 4 and 5, of amethod for specifying a beam used for communication in the radiocommunication system according to the present embodiment.

FIG. 4 is a diagram illustrating a configuration of a beam used forcommunication and each reference signal. FIG. 5 is a diagramillustrating a configuration of a beam used for communication.

As illustrated in FIG. 4, the reference signal in this embodiment has ahierarchical structure. That is, the user apparatus 20 sequentiallyrefers to a reference signal of a lower hierarchy from a referencesignal of a higher hierarchy to detect an optimal beam, among multiplebeams transmitted from the small base stations 11 and 12, which enablesthe user apparatus 20 to receive a desired data signal with the optimalbeam. By employing such a hierarchical structure, candidate beams may beefficiently extracted without searching for a very large number of beamreference signals of the corresponding beams. Accordingly, it ispossible to quickly detect or switch the optimum beam.

More specifically, in the example illustrated in FIG. 4, macro auxiliaryinformation is provided as a first layer (First Layer). As describedabove, the macro auxiliary information is transmitted from the macrobase station 10 to the user apparatus 20 within the macro coverage, andthe user apparatus 20 receives the discovery signal using the macroauxiliary information.

The macro auxiliary information includes, for example, information(timing, frequency, etc.) of a radio resource by which a discoverysignal is transmitted, discovery signal sequence information, and thelike. The channel via which the macro base station 10 transmits themacro auxiliary information is not limited to a specific type ofchannel, and may be transmitted using, for example, a control channel, abroadcast channel, or a data channel defined by LTE.

The radio communication system includes multiple macro base stations,and the user apparatus 20 may be enabled to obtain, by referring to themacro auxiliary information, information about the discovery signal tobe searched for and to have high possibility of being received withinthe macrocell in which the user apparatus 20 is located. That is, theuser apparatus 20 may be enable to specify a branch A in FIG. 4 byreferring to the macro auxiliary information. Since the macro auxiliaryinformation is a signal being referred to (Referenced) by the userapparatus 20, the macro auxiliary information may be called a type ofreference signal.

In the example illustrated in FIG. 4, a discovery signal is provided asa second hierarchy (Second Layer). The discovery signal is a precodedsignal that is transmitted from the small base stations 11 and 12 withboth a beam transmitted with a wide width (hereinafter referred to as a“wide beam”) and a beam transmitted with a width narrower than the widebeam (hereinafter referred to as a “narrow beam”. The wide beamindicates multiple beams transmitted in a relatively wide range asillustrated, for example, on the left side of FIG. 5. The narrow beamindicates multiple beams transmitted with a width narrower than that ofthe wide beam as illustrated, for example, in the center of and on theright side of FIG. 5.

A discovery signal is transmitted from each of the small base stations11 and 12 in each of multiple wide beams and multiple narrow beams, andthe user apparatus 20 monitors and receives (detects) the discoverysignal included in the wide beam based on the macro auxiliaryinformation. Upon detecting the discovery signal included in the widebeam, the user apparatus 20 subsequently searches for a discovery signalcorresponding to a narrow beam. Upon receiving (detecting) the discoverysignal corresponding to the narrow beam, the user apparatus 20 measuresthe reception quality (reception power, etc.) of the discovery signalcorresponding to the detected one or more narrow beams and transmits themeasured result to the base stations. In this embodiment, unlessotherwise specified, the term “reception quality” is used to include“reception power” in a broad sense.

In the example of FIG. 4, for example, in the second hierarchy, thediscovery signal included in the wide beam enables the selection of abranch B (branches Bs) residing in a lower hierarchy of the branch A(each branch B corresponds to a different wide beam). Likewise, thediscovery signal included in the narrow beam enables the selection of abranch C (branches Cs) residing in a lower hierarchy of thecorresponding branch B (each branch C corresponds to a different narrowbeam). Further, in the example of FIG. 5, the branch of B corresponds toeach of the wide beams depicted on the left side of FIG. 5, and thebranch of C corresponds to each of the narrow beams illustrated in thecenter of FIG. 5.

Information (a signal sequence or the like) on the discovery signal isassociated with each wide beam and each narrow beam. This informationmay be referred to as an “identifier”. That is, the user apparatus 20 isconfigured to measure the reception quality (reception power, etc.) of adiscovery signal associated with multiple different identifierstransmitted from the base stations. In the following description, anidentifier associated with a wide beam is referred to as a “beam groupID”, and an identifier associated with a narrow beam is referred to as a“beam ID”. The wide beam may be uniquely identified with the beam groupID, and the narrow beam may be uniquely identified with the beam ID.

In the example illustrated in FIG. 4, a measurement reference signal isprovided as a third hierarchy (Third Layer). A measurement referencesignal is a signal transmitted from the small base stations 11 and 12with a narrow beam. The small base stations 11 and 12 determine a narrowbeam for transmitting a measurement reference signal based on thereception quality (reception power, etc.) of the discovery signalcorresponding to one or more narrow beams reported from the userapparatus 20. For example, the small base stations 11 and 12 may beenabled to determine each of the narrow beams for transmittingmeasurement reference signals in the order from highest reception powerto lowest reception power of discovery signals.

Subsequently, the small base stations 11 and 12 transmit measurementreference signals for each of the determined narrow beams. The userapparatus 20 receives each measurement reference signal, measuresreception quality (reception power, etc.), and transmits feedbackinformation (e.g., identification information of the measurementreference signal having the highest reception power, etc.) to the smallbase stations 11 and 12, respectively. The small base stations 11 and 12that have received the feedback information, perform link adaptation,rank adaptation, scheduling, and the like with respect to a downlinkdata signal.

In the example illustrated in FIG. 4, when the small base station 11selects two branches each indicated by “D” as narrow beams fortransmitting measurement reference signals, the user apparatus 20measures reception quality of the measurement reference signals includedin the two narrow beams, and transmits feedback information to the smallbase station 11. The user apparatus 20 receives data signals transmittedfrom the small base station 11 via the two narrow beams. As illustratedon the right side of FIG. 5, the user apparatus 20 is configured toreceive the data signals transmitted from the small base station 11 viatwo narrow beams (beams corresponding to respective Ds in FIG. 5).

Note that the small base stations 11 and 12 may be configured to furtherselect narrow beams used for communication with the user apparatus 20based on the feedback information received from the user apparatus 20(e.g., further select a narrow beam from the beams corresponding torespective Ds in FIG. 5).

(Configuration Example of Each Signal)

In the present embodiment, a discovery signal included in a wide beammay be used for the user apparatus 20 to synchronize with the small basestations 11 and 12. Hence, in the present embodiment, the discoverysignal included in the wide beam may also be referred to as a“synchronization signal”. For example, PSS (Primary Synchronizationsignal)/SSS (Secondary Synchronization signal) may each be used as adiscovery signal included in a wide beam.

In the present embodiment, a discovery signal corresponding to a narrowbeam may, for example, be a predetermined reference signal, because theuser apparatus 20 is used to measure the reception quality for eachnarrow beam.

FIG. 6 is a diagram illustrating an example of mapping of discoverysignals in an embodiment. As illustrated in FIG. 6, for example, PSS andSSS used as discovery signals included in the wide beams are mapped tothe center of the system band, and the discovery signals correspondingto the narrow beams are mapped above and below PSS and SSS. In theexample of FIG. 6, the discovery signals mapped above and below PSS andSSS have the same (signal) sequence.

An example of signal mapping in each block (a time length is 1 symbol)is depicted on the right side of FIG. 6. In the example on the rightside of FIG. 6, the number indicates a beam (a discovery signal of thenarrow beam) for transmitting a signal with the corresponding resource.In the example depicted on the right side of FIG. 6, the discoverysignals of eight narrow beams are distributed and mapped to subcarriers.Note that the discovery signals in the embodiment are not limited to theexample illustrated in FIG. 6. The discovery signals in the embodimentmay employ a signal or a mapping method differing from those depicted inFIG. 6.

In the present embodiment, a CSI-RS (Channel State Information-ReferenceSignal) may be used as a measurement reference signal.

The outline of the mobile communication system according to the presentembodiment has been described above with reference to FIGS. 4 to 6. Inthe embodiment, as illustrated above in the problem to be solved by theinvention, in order to achieve communication by MIMO by simultaneouslyusing multiple beams, narrow beams (corresponding to “Ds” in FIG. 5) maybe selected so as to acquire a low correlation between the two narrowbeams illustrated in FIGS. 4 and 5 (i.e., so as to spatially separatethe two narrow beams). Specifically, when measuring the receptionquality of the discovery signals associated with the narrow beamscorresponding to the branch C in FIGS. 4 and 5, the user apparatus 20identifies directions in which the narrow beams are received, groups themeasured reception qualities into groups of the identified directions inwhich the narrow beams are received, and transmits the measuredreception quality by each of the groups to the small base stations 11and 12.

Further, when determining the narrow beams for transmitting themeasurement reference signals, the small base stations 11 and 12 do notsimply determine the multiple narrow beams in a descending order ofreception quality, but determine the multiple narrow beams so as todistribute the determined narrow beams into groups of differentdirections. The low correlation between the multiple narrow beams usedfor communication may be acquired by determining the multiple narrowbeams for transmitting the measurement reference signals in a mannerdescribed above. Hence, it is possible to improve throughput or the likeby MIMO spatial multiplexing or space division multiplexing.

In the following description, the small base stations 11 and 12 arecollectively referred to as a “small base station 11”.

<Functional Configuration>

(User Apparatus)

FIG. 7 is a diagram illustrating an example of a functionalconfiguration of a user apparatus according to an embodiment. Asillustrated in FIG. 7, the user apparatus 20 includes a signal receivingunit 101, a signal transmission unit 102, a reception quality measuringunit 103, and a feedback information generating unit 104. The signalreceiving unit 101 includes a reception direction detecting unit 111.FIG. 7 merely illustrates the functional configuration particularlyrelated to the embodiment of the present invention in the user apparatus20, and the user apparatus 20 may also include not-illustrated functionsfor performing operations at least in compliance with LTE. Thefunctional configuration of the user apparatus 20 illustrated in FIG. 7is merely an example. Any functional division and any names of thefunctional components may be applied insofar as operations according tothe present embodiment may be executable.

The signal receiving unit 101 acquires information of an upper layerfrom wirelessly received signal(s) of a lower layer. The signalreceiving unit 101 acquires macro auxiliary information from controlsignal(s) and the like received from the macro base station 10, storesthe macro auxiliary information, and receives (detects), based on thestored macro auxiliary information, discovery signal(s) included in awide beam transmitted from the small base station 11. Further, thesignal receiving unit 101, for example, specifies a signal sequencesimultaneously with performing symbol timing synchronization and radioframe synchronization using the discovery signals included in the widebeam, and receives (detects) discovery signal(s) corresponding to thenarrow beam in accordance with the specified signal series. The signalreceiving unit 101 also receives measurement reference signal(s)transmitted from the small base station 11.

The signal transmission unit 102 generates a signal of a lower layerfrom the information of the upper layer and transmits the generatedsignal of the lower layer wirelessly. Further, the signal transmissionunit 102 transmits the reception quality measured by the receptionquality measuring unit 103 to the small base station 11. Further, thesignal transmission unit 102 transmits the feedback informationgenerated by the feedback information generating unit 104 to the smallbase station 11.

The reception quality measuring unit 103 measures reception quality(reception power, etc.) of discovery(s) signal corresponding to a narrowbeam. More specifically, the reception quality measuring unit 103specifies a beam ID of each narrow beam from the discovery signalscorresponding to the narrow beam, measures the reception quality of thenarrow beam corresponding to the specified beam ID, and transmits themeasured reception signal to the signal transmission unit 102. Inaddition, the reception quality measuring unit 103 acquires, from thereception direction detecting unit 111, a reception direction of thenarrow beam, a specific directivity pattern in which the narrow beam isreceived, or a specific antenna port via which the narrow beam isreceived, or the like so as to obtain information on a receptiondirection of the narrow beam of which reception quality is to bemeasured. Note that the reception quality measuring unit 103 measuresthe reception quality (reception power or the like) of the discoverysignals corresponding to the narrow beam, based on an instruction fromthe small base station 11 or at a predetermined period, and transmitsthe measured reception quality via the signal transmission unit 102 tothe small base station 11.

The feedback information generating unit 104 generates feedbackinformation based on a measured result obtained based on the measurementreference signals corresponding to the narrow beam, and transfers thegenerated feedback information to the signal transmission unit 102. Thefeedback information may include CSI such as Rank Indicator (RI),Precoding Matrix Indicator (PMI), Channel Quality Indicator (CQI), orthe like. Note that the feedback information generating unit 104 maytransfer the feedback information relating to all the narrow beamsincluding the measurement reference signals to the signal transmissionunit 102 or may transfer the feedback information on a predeterminednumber of narrow beams in a descending or an ascending order of thereception quality of the narrow beams to the signal transmission unit102. Note that the feedback information generating unit 104 generatesfeedback information, based on an instruction from the small basestation 11 or by measuring the measurement reference signal included inthe narrow beam at a predetermined period, and transmits the generatedfeedback information via the signal transmission unit 102 to the smallbase station 11.

The reception direction detecting unit 111 forms a beam on a receivingside by using multiple antennas held by the user apparatus 20 to detecta direction in which the narrow beam is received. Further, the receptiondirection detecting unit 111 reports the detected direction of thedetected narrow beam to the reception quality measuring unit 103.Forming a beam on the receiving side indicates that the received signalis multiplied by the weight for each antenna port of the user apparatus20 to generate a directivity pattern such that a narrow beam is receivedwith a certain width (i.e., with directivity). In addition, thereception direction detecting unit 111 may report to the receptionquality measuring unit 103, a specific directivity pattern in which thenarrow beam is received, instead of the reception direction of thenarrow beam. The weight to be multiplied for each antenna port may bespecified by a code book.

Further, the reception direction detecting unit 111 may be configured todetect the reception direction of the narrow beam by specifying, fromamong the multiple antenna ports included in the user apparatus 20, anantenna port via which the narrow beam is received. This is because itis also assumed that the reception direction may be specified by a shapeof the physical antenna provided in the user apparatus 20. In such acase, the reception direction detecting unit 111 may report, instead ofthe reception direction of the narrow beam, a specific antenna port viawhich the narrow beam is received to the reception quality measuringunit 103.

(Small Base Station)

FIG. 8 is a diagram illustrating an example of a functionalconfiguration of a small base station according to an embodiment. Asillustrated in FIG. 8, a small base station 11 includes a signalreceiving unit 201, a signal transmission unit 202, a candidate beamselecting unit 203, and a beam control unit 204. FIG. 8 merelyillustrates the functional configuration particularly related to theembodiment of the present invention in the small base station 11, andthe small base station 11 may also include not-illustrated functions forperforming operations at least in compliance with LTE. The functionalconfiguration of the small base station 11 illustrated in FIG. 8 ismerely an example. Any functional division and any names of thefunctional components may be applied insofar as operations according tothe present embodiment may be executable.

The signal receiving unit 201 acquires information of an upper layerfrom wirelessly received signal(s) of a lower layer. In addition, thesignal receiving unit 201 transfers, to the candidate beam selectingunit 203, the reception quality of each narrow beam received from theuser apparatus 20. The signal receiving unit 201 also transfers, to thebeam control unit 204, the feedback information of the narrow beamreceived from the user apparatus 20.

The signal transmission unit 202 generates a signal of a lower layerfrom the information of the upper layer and transmits the generatedsignal of the lower layer wirelessly. In addition, the signaltransmission unit 202 transmits a radio signal so as to form a wide beamand a narrow beam including a discovery signal by beamforming, which isachieved by multiplying each antenna port by a predetermined weight. Inaddition, the signal transmission unit 202 transmits, based ondesignation from the candidate beam selecting unit 203, the measurementreference signal from the specified narrow beam. Further, based on thespecification made by the beam control unit 204, the signal transmissionunit 202 assigns, in response to the designated narrow beam, radioresources and the like for transmitting a data signal (e.g., PDSCH orthe like) to the user apparatus 20.

The candidate beam selecting unit 203 selects one or more candidatenarrow beams to be used for communication with the user apparatus 20based on the reception quality of each of the narrow beams reported fromthe user apparatus 20 for a corresponding one of reception directions.For example, the candidate beam selecting unit 203 may select one ofnarrow beams with the best reception quality for each of the receptiondirections, or may select a predetermined number of narrow beams indescending order of reception quality for each of the receptiondirections. Further, in a case where narrow beams with good receptionquality are present only in a specific reception direction, apredetermined number of narrow beams may be selected in descending orderof reception quality, from among narrow beams received from a specificreception direction. Various methods or techniques may be considered forselecting one or more narrow candidate beams used for communication withthe user apparatus 20. That is, the candidate beam selecting unit 203may select candidate beams not only by employing the selection methoddescribed above, but also the candidate beam selecting unit 203 mayselect the candidate narrow beams by other selection methods.

In addition, the candidate beam selecting unit 203 instructs the signaltransmission unit 202 to transmit the measurement reference signal(s)from one or more selected narrow beams. For example, the candidate beamselecting unit 203 may select antenna ports included in the small basestation 11 as well as determining the weight to be multiplied for eachantenna port so as to instruct the signal transmission unit 202 totransmit the measurement reference signal(s) from the selected one ormore narrow beams.

The beam control unit 204 controls the antenna ports included in thesmall base station 11 based on the feedback information of the narrowbeams received from the user apparatus 20 to thereby achieve MIMOspatial multiplexing using multiple narrow beams, transmissiondiversity, and the like. Further, the beam control unit 204 maysequentially switch (optimize) narrow beams used for communicationaccording to traveling of the user apparatus 20 (performs beam tracking)based on feedback information of the narrow beam received from the userapparatus 20. Note that the above-described controls may be performed bylinkage between the beam control unit 204 and the signal transmissionunit 202.

The functional configurations of the user apparatus 20 and the smallbase station 11 may be entirely implemented by one, or two or morehardware circuits (e.g., one IC chip or multiple IC chips).Alternatively, a part of functional configurations of the small basestation 11 and the user apparatus 20 may be formed by hardware circuits,and the remaining part may be implemented by a CPU and a program.

(User Apparatus)

FIG. 9 is a diagram illustrating an example of a hardware configurationof a user apparatus according to an embodiment. FIG. 9 illustrates aconfiguration closer to the implemented example than the exampleillustrated in FIG. 7. As illustrated in FIG. 9, a user apparatus 20includes an RF (Radio Frequency) module 301 configured to perform aprocess relating to radio signals, a BB (Base Band) process module 302configured to perform a baseband signal process, and a UE control module303 configured to perform a process of an upper layer and the like.

The RF module 301 performs D/A (Digital-to-Analog) conversion,modulation, frequency conversion, power amplification, etc., on adigital baseband signal received from the BB process module 302 togenerate a radio signal to be transmitted from an antenna. The RF module301 also performs frequency conversion, A/D (Analog to Digital)conversion, demodulation, etc., on the received radio signal to generatea digital baseband signal to transfer the generated digital basebandsignal to the BB process module 302. The RE module 301 includes, forexample, a part of the signal receiving unit 101 and a part of thesignal transmission unit 102 illustrated in FIG. 7.

The BB process module 302 is configured to perform a process of mutuallyconverting an IP packet and a digital baseband signal. A DSP (DigitalSignal Processor) 312 is a processor configured to perform a signalprocess in the BB process module 302. The memory 322 is used as a workarea of the DSP 312. The BB process module 302 includes, for example, apart of the signal receiving unit 101, a part of the signal transmissionunit 102, a reception quality measuring unit 103, and a feedbackinformation generating unit 104 illustrated in FIG. 7.

The UE control module 303 is configured to perform an IP layer protocolprocess, various types of application processes, and the like. Theprocessor 313 is configured to perform a process performed by the UEcontrol module 303. The memory 323 is used as a work area of theprocessor 313.

(Small Base Station)

FIG. 10 is a diagram illustrating an example of a hardware configurationof a small base station according to an embodiment. FIG. 10 illustratesa configuration closer to the implemented example than the exampleillustrated in FIG. 8. As illustrated in FIG. 10, the small base station11 includes an RF module 401 configured to perform a process relating toradio signals, a BB process module 402 configured to perform a basebandsignal process, an apparatus control module 403 configured to perform aprocess of a higher layer and the like, and a communication IF 404serving as an interface for connecting to a network.

The RF module 401 performs D/A conversion, modulation, frequencyconversion, power amplification, etc., on a digital baseband signalreceived from the BB process module 402 to generate a radio signal to betransmitted from an antenna. The RF module 401 also performs frequencyconversion, A/D conversion, demodulation, etc., on the received radiosignal to generate a digital baseband signal to transfer the generateddigital baseband signal to the BB process module 402. The RF module 401includes, for example, a part of the signal receiving unit 201 and apart of the signal transmission unit 202 illustrated in FIG. 8.

The BB process module 402 is configured to perform a process of mutuallyconverting an IP packet and a digital baseband signal. A DSP (DigitalSignal Processor) 412 is a processor configured to perform a signalprocess in the BB process module 402. The memory 422 is used as a workarea of the DSP 412. The BB process module 402 includes, for example, apart of the signal receiving unit 201, a part of the signal transmissionunit 202, a part of the candidate beam selecting unit 203, and a part ofthe beam control unit 204 illustrated in FIG. 8.

The apparatus control module 403 is configured to perform an IP layerprotocol process, an OAM (Operation and Maintenance) process, and thelike. The processor 413 is configured to perform a process performed bythe apparatus control module 403. The memory 423 is used as a work areaof the processor 413. An auxiliary storage device 433 may, for example,be an HDD or the like, and is configured to store various configurationinformation and the like for the small base station 11 itself tooperate. The apparatus control module 403 includes, for example, a partof the candidate beam selecting unit 203 and a part of the beam controlunit 204 illustrated in FIG. 8.

<Process Flow>

(Measuring Method and Reporting Method of Reception Quality)

FIG. 11 is a diagram illustrating a method for measuring a discoverysignal according to an embodiment. As illustrated in FIG. 11, the userapparatus 20 measures, with respect to narrow beams arriving fromdifferent reception directions (A to D in FIG. 11), reception quality ofa discovery signal of the narrow beam(s) for each of the receptiondirections, and reports measured results to the small base station 11.

The user apparatus 20 groups one or more narrow beams arriving from eachof the reception directions into groups by reception direction. Forexample, the user apparatus 20 groups one or more narrow beams receivedfrom the reception direction A into a group as a beam set #1, groups oneor more narrow beams received from the reception direction B into agroup as a beam set #2, and groups one or more narrow beams receivedfrom the reception direction C into a group as a beam set #3. Althoughnot illustrated in FIG. 11, if there is one or more narrow beamsreceived from the reception direction D, these narrow beams are groupedinto a group as a beam set #4.

By grouping narrow beams in the above-described manner, the small basestation 11 may be enabled to determine whether the user apparatus 20 hasreceived the reception quality of the discovery signal of each narrowbeam in the same reception direction or in a different receptiondirection.

Note that the user apparatus 20 does not necessarily have to associatethe reception directions A to D with the beam sets #1 to #4,respectively. For example, the user apparatus 20 may associate the beamset #1 with a reception direction in which narrow beams exhibit the bestreception quality (reception power, etc.) among the discovery signals ofall the narrow beams received from different reception directions.Further, the user apparatus 20 may associate the beam set #2 with areception direction in which narrow beams exhibit a good receptionquality (reception power, etc.). This is because even if the small basestation 11 does not particularly identify a reception direction in whichthe narrow beam is received (e.g., whether the narrow beam is receivedin an upper direction or a right direction of a terminal), the smallbase station 11 capable of determining that the narrow beams arereceived in different reception directions may be enabled to selectcandidate narrow beams for use in downlink communication.

FIG. 12 is a diagram illustrating the number of narrow beams of whichthe user apparatus reports the reception quality. For example, the userapparatus 20 may measure the reception quality of the discovery signalcorresponding to one or more narrow beams grouped by receptiondirection, and may report, among the measured one or more receptionqualities, only the reception quality that satisfies a predeterminedcondition, to the small base station 11. The user apparatus 20 may beenabled to reduce a data amount of signals to be transmitted from theuser apparatus 20 to the small base station 11 by not reporting thereception quality associated with the narrow beams exhibiting poorreception quality (i.e., narrow beams that are unlikely to be finallyused for communication).

The following illustrates specific examples of such a case withreference to FIGS. 11 and 12. In FIG. 12, a horizontal axis represents abeam ID uniquely identifying a narrow beam, and a vertical axisrepresents the magnitude of RSRP (Reference Signal Received Power). Trepresents a predetermined range. For example, FIG. 12 depicts anexample of a measured result obtained by the user apparatus 20 that hasmeasured the reception power (RSRP) of the discovery signalcorresponding to multiple narrow beams received from the receptiondirection A in FIG. 11. For example, the user apparatus 20 selects anarrow beam (“A” in FIG. 12) having the highest reception power, andfurther selects narrow beams (“B” to “E” in FIG. 12) having thereception power within the predetermined range “T” from the receptionpower of the selected narrow beam “A”. Subsequently, the user apparatus20 transmits the reception power relating to the selected narrow beams Ato E and the beam ID of each of the narrow beams to the small basestation 11 together with information for identifying the group of thebeam set #1.

Note that the predetermined range “T” may be set in advance in the userapparatus 20, or may be reported to the user apparatus 20 in advanceusing a control signal of a macrocell or broadcast information and thelike.

Further, the user apparatus 20 may measure the reception quality of thediscovery signal corresponding to one or more narrow beams grouped byreception direction, and may report only a predetermined number ofreception qualities, among the measured one or more reception qualities,to the small base station 11. For example, when the predetermined numberis “5”, the user apparatus 20 transmits, to the base station 11, thereception quality relating to the five narrow beams in descending orderof the reception quality, among the reception qualities relating to themultiple narrow beams received from the reception direction A.

Further, the user apparatus 20 may restrict the upper limit of thenumber of the overall reception qualities to be transmitted to the smallbase station 11. For example, the reception quality relating to the fivenarrow beams may be measured for a group of the beam set #1, thereception quality relating to six narrow beams may be measured for agroup of the beam set #2, and the reception quality relating to sevennarrow beams may be measured for a group of the beam set #3. In a casewhere the upper limit of the number of the reception qualities to betransmitted is 10, the user apparatus 20 may transmit, to the small basestation 11, only the reception quality relating to 10 narrow beams indescending order of reception quality among the reception qualityrelating to those 18 narrow beams. According to the above-describedconfiguration, it may be possible to reduce the data amount of thesignals transmitted from the user apparatus 20 to the small base station11.

(Processing Sequence)

FIG. 13 is a diagram illustrating an example of a process sequence of amobile communication system according to an embodiment. The followingillustrates, with reference to FIG. 13, a sequence of processes fromdiscovery of a wide beam and a narrow beam by the user apparatus 20 todata transmission from the small base station 11 to the user apparatus20. Note that in the following example, it is assumed that the userapparatus 20 has received macro auxiliary information from a macro basestation in advance or holds auxiliary information corresponding to macroauxiliary information in advance. That is, the user apparatus 20 hasobtained, in advance, information (frequency band, bandwidth,transmission timing of each discovery signal, series, etc.) fordiscovering a wide beam, and information relating to transmission timingand series etc., of a discovery signal corresponding to a narrow beam.

In step S301, the signal transmission unit 202 of the small base station11, transmits, multiple pre-coded discovery signals forming a wide beamand multiple pre-coded discovery signals forming a narrow beam asdescribed above.

In step S302, the signal receiving unit 101 of the user apparatus 20receives discovery signals included in the wide beam based on the macroauxiliary information or auxiliary information corresponding to themacro auxiliary information, and performs frequency synchronization withthe small base station 11, as well as performing timing synchronization(symbol synchronization, frame synchronization, etc.). In accordancewith the discovery signals included in the wide beam, the user apparatus20 may receive information (minimum system information, etc.) necessaryfor communication in the coverage of the small base station 11.Subsequently, the user apparatus 20 searches for discovery signalscorresponding to the narrow beam according to the signal sequencespecified by the discovery signals included in the wide beam to specifythe beam ID of the narrow beam capable of receiving the discoverysignals. For example, in a case where the discovery signal correspondingto the narrow beam has a format as illustrated in FIG. 6, the signalreceiving unit 101 may be able to specify the beam ID, based on areceivable narrow beam OFDM symbol and a position of the subcarrier.

Subsequently, the reception quality measuring unit 103 of the userapparatus 20 measures the reception quality of the discovery signalscorresponding to the receivable narrow beam. In this case, the receptionquality measuring unit 103 may receive, from the reception directiondetecting unit 111, the reception direction of the discovery signals ofthe detected narrow beam, or the reception direction detecting unit 111may extract the reception direction to search for the discovery signalscorresponding to the narrow beam. As a result, the user apparatus 20 maybe enabled to specify the beam ID of the narrow beam receivable in eachreception direction, and be enabled to measure the reception quality ofthe discovery signals corresponding to each narrow beam.

In step S303, the signal transmission unit 202 of the small base station11 transmits, to the user apparatus 20, the information on the allocatedradio resources in order for the user apparatus 20 to transmit thereception quality of the discovery signals corresponding to each narrowbeam. Note that the process of step S303, may be performed by allowingthe user apparatus 20 to request the small base station 11 to allocateuplink radio resources, for example, by a random access procedure or viaPUCCH (Physical Uplink Control Channel), EPUCCH (Enhanced PhysicalUplink Control Channel), and the like. Alternatively, the process ofstep S303 may be performed by allowing the user apparatus 20 to requestthe small base station 11 to allocate uplink radio resources, forexample, via a macro base station. The small base station 11 mayautonomously perform uplink radio resource allocation according totiming of transmitting the discovery signals in step S301.

In step S304, the signal transmission unit 102 of the user apparatus 20reports the reception quality of the discovery signals corresponding tothe narrow beam measured in step S302 by transmitting a receptionquality report signal to the small base station 11. An uplink physicalcontrol channel (PUCCH/EPUCCH) or a random access channel (PRACH)defined in LTE may be used as an uplink channel via which the signaltransmission unit 102 of the user apparatus 20 transmits the receptionquality report signal; however, the uplink channel for transmitting thereception quality report signal is not restricted to these examples.

The following describes a signal format for use in the reception qualityreport signal.

FIG. 14 is a diagram illustrating an example of a configuration of areception quality report signal. As illustrated in FIG. 14, thereception quality report signal includes “format information”, “beam IDand reception quality (RSRP)” for each of the discovery signalscorresponding to each narrow beam, and “CRC (Cyclic Redundancy Check)”.

The “format information” is configured to store information includingeach of beam sets in association with the beam ID and the receptionquality (RSRP) of each of the discovery signals corresponding to each ofthe narrow beams. The “CRC” is calculated by masking (e.g., XOR masking)a CRC calculated based on the format information and the beam ID andreception quality (RSRP) of the discovery signals corresponding to thenarrow beams with the UE ID (e.g., C-RNTI: Cell-Radio Network TemporaryIdentifier) that identifies the user apparatus 20.

That is, the “format information” corresponds to header information andthe “beam ID and reception quality (RSRP)” corresponds to a dataportion.

Note that the signal transmission unit 102 of the user apparatus 20encodes the reception quality report signal illustrated in FIG. 14 andtransmits the encoded reception quality report signal to the userapparatus 20. In this example, a joint coding method that encodes the“format information”, “beam ID and reception quality (RSRP)” and “CRC”as a set may be considered as a coding method.

Further, a separate coding method that separately encodes the “formatinformation”, “beam ID and reception quality (RSRP)”, and “CRC”respectively may be considered as an alternative coding method. Withthis method, the small base station 11 may be enabled to refer to theheader information (“format information”) separately from the dataportion (“beam ID and reception quality (RSRP)”).

FIG. 15 includes diagrams illustrating an example of format information.As illustrated in FIG. 15, the format information may have severalformats. The format information of FIG. 15(a) stores “number of beamsets” first, and subsequently stores the “number of beams in the beamset #N” repeatedly according to the number of beam sets. For example, inthe format information of FIG. 15 (a), when the “number of beam sets” is2, the “number of beams in the beam set #1” is 2, and the “number ofbeams in the beam set #2” is 3, among five “beam IDs and receptionqualities” subsequently stored subsequently to the format information,the first two “beam IDs and reception qualities” are information on beamset #1, and the next three “beam IDs and reception qualities” areinformation on beam set #2. According to the format information of FIG.15(a), the size of format information may increase; however, the formatinformation of FIG. 15(a) has an advantage that there is no limitationin the number of beam sets to be stored.

Next, the format information of FIG. 15(b) stores the “number of beamsin the beam set #1” and the “number of beams in the beam set #2”. In acase where the format information of FIG. 15(b) is employed, the userapparatus 20 may only report information on two beam sets; however, theuser apparatus 20 has an advantage of preventing the format size frombecoming unlimitedly large. Note that in a case where the formatinformation of FIG. 15(b) is employed, and there is only one beam set tobe reported from the user apparatus 20 (i.e., when narrow beams arereceived only in a specific reception direction), the user apparatus 20may set “zero” to the “number of beams in the beam set #2”. Further, theformat information of FIG. 15(b) may be configured to store informationon three or more beam sets. For example, the format information of FIG.15(b) may be configured to store the “number of beams in the beam set#1”, the “number of beams in the beam set #2” and the “number of beamsin the beam set #3”.

Subsequently, the format information of FIG. 15(c) stores the “totalnumber of beams” and the “number of beams in the beam set #1”. In theformat information of FIG. 15(c), the number of beams in set #2 may beobtained by calculation of ‘the “total number of beams”−the “number ofbeams in the beam set #1”’. Further, when the same number is set in eachof the “total number of beams” and the “number of beams in the beam set#1”, there is only one beam set reported from the user apparatus 20.Similar to the example of FIG. 15(b), the format information of FIG.15(c) has an advantage of preventing the format size from becomingunlimitedly large although the user apparatus 20 may only reportinformation on two beam sets.

Subsequently, the format information of FIG. 15(d) stores the “number ofbeam sets” and the “total number of beams”. For example, when the“number of beam sets” is 3 and the “total number of beams” is 9, thereception quality report signal includes three “beam IDs and receptionqualities” are stored for each of the beam set. In a case where theformat information of FIG. 15(d) is employed, the user apparatus 20needs to apply the same number of “beam IDs and reception qualities” tobe reported for each of the beam sets. In this case, the user apparatus20 has an advantage of being able to report information on multiple beamsets as well as having an advantage of preventing the format size frombecoming unlimitedly large.

Subsequently, a modification of a signal format for use in the receptionquality report signal will be described. FIG. 16 includes diagramsillustrating a modification of a configuration of the reception qualityreport signal. As illustrated in FIG. 16, the reception quality reportsignal includes the “format information”, the “beam set ID” foridentifying each of the beam sets (#1 to #N), the “beam ID and receptionquality (RSRP)” for each of the discovery signals corresponding to eachof the narrow beams, and the “CRC”.

The “format information” is configured to store information indicatinghow many of the beam IDs and the reception qualities (RSRP) of each ofthe discovery signals corresponding to each of the narrow beams arestored in the reception quality report signal. The “CRC” is calculatedby masking (e.g., XOR masking) a CRC calculated based on the formatinformation, the beam set ID, and the beam ID and reception quality(RSRP) of each of the discovery signals corresponding to each narrowbeam with the UE ID (e.g., C-RNTI:) that identifies the user apparatus20.

That is, the “format information” corresponds to header information andthe “beam set ID” and “beam ID and reception quality (RSRP)” correspondto a data portion.

Note that the signal transmission unit 102 of the user apparatus 20encodes the reception quality report signal illustrated in FIG. 16 andtransmits the encoded reception quality report signal to the userapparatus 20. In this example, a joint coding method that encodes the“format information”, “beam set ID”, “beam ID and reception quality(RSRP)” and “CRC” as a set may be considered as a coding method.

Further, a separate coding method that separately encodes the “formatinformation”, “beam ID and reception quality (RSRP)”, and “CRC”respectively may be considered as an alternative coding method. Withthis method, the small base station 11 may be enabled to refer to theheader information (“format information”) separately from the dataportion (“beam set ID” and “beam ID and reception quality (RSRP)”).

FIG. 17 includes diagrams illustrating an example of a modification ofthe format information. The format information illustrated in FIG. 17may be applied, for example, when the reception quality report signalhas a configuration as illustrated in FIG. 16. As illustrated in FIG.17, the format information may have several formats. The formatinformation of FIG. 17(a) may first store the “total number of beams”.For example, in the format information of FIG. 17(a), when “the totalnumber of beams” is 4, the number of pairs of “beam set ID” and “beam IDand reception quality” stored following the format information is 4. Themodified example of FIG. 17(a) has an advantage of preventing the sizeof the format information from becoming unlimitedly large.

Subsequently, the format information of FIG. 17(d) stores the “totalnumber of beams” and the “number of beam sets”. For example, in theformat information of FIG. 17(b), in a case where the “total beamnumber” is 4 and the “number of beam sets” is 2, the number of pairs of“beam set ID” and “beam ID and reception quality” stored subsequently tothe format information is 4, and the “beam ID and reception quality” ineach of the two beam sets is included in the reception quality reportsignal. Similar to the modified example of FIG. 17(a), the formatinformation of FIG. 17(b) has an advantage of preventing the size of theformat information from becoming unlimitedly large. The subsequentillustration is given below by referring back to FIG. 13.

In step S305, the candidate beam selecting unit 203 of the small basestation 11 selects one or more candidate beams to be used forcommunication with the user apparatus 20, based on the reception qualityof the discovery signal corresponding to the narrow beam stored in thereception quality report signal received in step S304. In this case, thecandidate beam selecting unit 203 sequentially selects the narrow beamsone by one from each of the beam sets included in the reception qualityreport signal in order of the reception quality, that is, in order fromone having the best reception quality (reception power, etc.).Specifically, the candidate beam selecting unit 203 selects one narrowbeam with the best reception quality from the multiple narrow beamsreported as the beam set #1, and subsequently selects one narrow beamwith the best reception quality from the multiple narrow beams reportedas the beam set #2. The candidate beam selecting unit 203 may selectthree or more narrow beams by repeating such processes.

Note that the candidate beam selecting unit 203 may further use an SRS(Sounding Reference Signal) received from the user apparatus 20 whenselecting one or more narrow candidate beams used for communication withthe user apparatus 20.

In step S306, the signal transmission unit 202 of the small base station11 transmits the measurement reference signal via the narrow beamselected in step S305.

In step S307, the feedback information generating unit 104 of the userapparatus 20 generates feedback information based on the measured resultobtained from the measurement reference signal corresponding to eachnarrow beam. The feedback information may, as described above, include aCSI such as Rank Indicator (RI), Precoding Matrix Indicator (PMI),Channel Quality Indicator (CQI), or the like.

In step S308, the signal transmission unit 202 of the small base station11 transmits, to the user apparatus 20, the information on the allocatedradio resources in order for the user apparatus 20 to transmit thefeedback information. Note that in step S308, the user apparatus 20 mayrequest the small base station 11 to allocate the uplink radio resourceby the random access procedure or PUCCH, EPUCCH, etc., in a mannersimilar to step S303, for example. The small base station 11 mayautonomously perform uplink radio resource allocation according totiming of transmitting the discovery signals in step S306.

In step S309, the signal transmission unit 102 of the user apparatus 20transmits the feedback information generated in step S307 to the smallbase station 11. An uplink physical control channel (PUCCH/EPUCCH) or arandom access channel (PRACH) defined in LTE may be used as an uplinkchannel via which the signal transmission unit 102 of the user apparatus20 transmits the feedback information; however, the uplink channel fortransmitting the feedback information is not restricted to theseexamples.

In step S310, the beam control unit 204 of the small base station 11determines a narrow beam used for transmitting downlink data to the userapparatus 20 based on the feedback information received in step S309.

In step S311, the beam control unit 204 and the signal transmission unit202 of the small base station 11 performs scheduling based on thefeedback information, determines a beam, rank, MCS (modulation andcoding scheme), resources, etc., suitable for the user apparatus 20, andappropriately controls the antenna port to transmit a data signal to theuser apparatus 20.

Note that in the mobile communication system according to theembodiment, when the reception direction of the discovery signal isdetected in steps S302 to S304 illustrated in FIG. 13, a specificdirectivity pattern in which a discovery signal is received, or aspecific antenna port via which the discovery signal is received may berecognized.

Note that in the mobile communication system according to theembodiment, steps S301 to S305 illustrated in FIG. 13 may be repeatedlyperformed at predetermined intervals. As a result, the small basestation 11 may be enabled to optionally change one or more narrowcandidate beams used for communication with the user apparatus 20.

Further, in the mobile communication system according to the embodiment,steps S306 to S311 illustrated in FIG. 13 may be repeatedly performed atpredetermined intervals. For example, the small base station 11 mayrepeat the process of steps S306 to S311 to optionally change (switch)the narrow beams used for transmitting the downlink data to the userapparatus 20 sequentially based on the feedback informationcorresponding to the multiple narrow beams. Further, the interval atwhich the process of steps S306 to S311 illustrated in FIG. 13 isrepeated and the interval at which the process of steps S306 to S311 isrepeated may be different time intervals.

In the mobile communication system according to the embodiment, theformat information may be transmitted to the user apparatus 20 togetherwith the radio resource allocation information in step S303 illustratedin FIG. 13. In this case, the signal transmission unit 202 of the smallbase station 11 may transmit the format information illustrated in FIG.18(a) to the user apparatus 20, and in step S304, the signaltransmission unit 102 of the user apparatus 20 may transmit, to thesmall base station 11, the reception quality report signal having theformat illustrated in FIG. 18(b) according to the format informationreceived in step S303. Note that the format information illustrated inFIG. 18(a) may be the same as the format information illustrated inFIGS. 15(a) to 15(d), for example.

Further, the user apparatus 20 and the small base station 11 may obtainin advance information on the configuration of the format information ofFIG. 18(a). The mobile communication system having such a configurationmay be enabled to save a time for transmitting the format informationbetween the user apparatus 20 and the small base station 11 as well asreducing the amount of data transmitted and received between the userapparatus 20 and the small base station 11.

In the mobile communication system according to another embodiment, theformat information may be transmitted to the user apparatus 20 togetherwith the radio resource allocation information in step S303 illustratedin FIG. 13. In this case, the signal transmission unit 202 of the smallbase station 11 may transmit the format information illustrated in FIG.19(a) to the user apparatus 20, and in step S304, the signaltransmission unit 102 of the user apparatus 20 may transmit, to thesmall base station 11, the reception quality report signal having theformat illustrated in FIG. 19(b) according to the format informationreceived in step S303. Note that the format information illustrated inFIG. 19(a) may be the same as the format information illustrated in FIG.17(a) or 17(b), for example.

Further, the user apparatus 20 and the small base station 11 may obtainin advance information on the configuration of the format information ofFIG. 19(a). The mobile communication system having such a configurationmay be enabled to save a time for transmitting the format informationbetween the user apparatus 20 and the small base station 11 as well asreducing the amount of data transmitted and received between the userapparatus 20 and the small base station 11.

In the mobile communication system according to still anotherembodiment, the format information may be transmitted to the userapparatus 20 together with the radio resource allocation information instep S303 illustrated in FIG. 13. In such a case, the signaltransmission unit 202 of the small base station 11 may transmit theformat information illustrated in FIG. 20(a) to the user apparatus 20,for example. In step S304, the signal transmission unit 102 of the userapparatus 20 may transmit a reception quality report signal having, forexample, a format illustrated in FIG. 20(b) to the small base station 11according to the format information received in step S303. The “totalnumber of beams” may be stored in the format information illustrated inFIG. 20(a), and the “number of beam sets” may be stored in the formatinformation illustrated in FIG. 20(b) For example.

(Transmission Method of Reception Quality Reporting Signal andTransmitting Feedback Information)

FIG. 21 includes diagrams illustrating a method for transmitting areception quality report signal and feedback information. The followingillustrates, with reference to FIG. 21, a transmission method when thereception quality report signal and the feedback information aretransmitted from the user apparatus 20 to the small base station 11.First, FIG. 21(a) gives schematic representation of how the receptionquality report signal and feedback information are separatelytransmitted. As illustrated in FIG. 21(a), compared to the feedbackinformation, the user apparatus 20 may transmit the reception qualityreport signal to the small base station 11 at a relatively long cycle.

Next, FIG. 21(b) gives schematic representation of how the receptionquality report signal and the feedback information are simultaneouslytransmitted at the timing at which the reception quality report signalis transmitted. As illustrated in FIG. 21(b), the signal transmissionunit 102 of the user apparatus 20 may transmit a reception qualityreport signal by including feedback information to the small basestation 11 at the timing at which the reception quality report signal istransmitted.

In the following, an illustration is given of a configuration example ofthe format information in a case where feedback information is includedin the reception quality report signal.

FIG. 22 includes diagrams illustrating an example of format informationto which a report pattern is attached. The format information itemsillustrated in FIGS. 22(a) to 22(d) each includes a “report pattern” inaddition to a corresponding one of the format information illustrated inFIGS. 15(a) to 15(d). The “report pattern” includes information (a flag)for identifying whether the reception quality report signal includesonly the reception quality of the discovery signal corresponding to thenarrow beam or whether the reception quality report signal includes boththe reception quality of the discovery signal corresponding to thenarrow beam and the feedback information.

The information may, for example, be composed of 2 bits so as toindicate four patterns. For example, in a case where “00” is set for the“report pattern”, the “report pattern” having the setting of “00” mayindicate that the reception quality report signal includes only thereception quality of the discovery signal corresponding to the narrowbeam. Likewise, in a case where “01” is set for the “report pattern”,the “report pattern” having the setting of “01” may indicate that thereception quality report signal includes both the reception quality ofthe discovery signal corresponding to the narrow beam and the feedbackinformation.

Note that each format information item illustrated in FIG. 22 may beused for reporting feedback information. For example, the formatinformation illustrated in FIG. 22 may be provided with a flag (e.g.,“10”) indicating that only the “feedback information” is included in the“report pattern” and the format information having such a flag may beadded to a header portion of the feedback information. In this case,zero may be stored in the “number of beam sets”, the “number of beams inbeam set #N” and the “total number of beams” included in the formatinformation. As a result, formats of the signals transmitted from theuser apparatus 20 to the small base station 11 may be standardized,thereby reducing processing load on the user apparatus 20 and the smallbase station 11.

Note that the above-described configurations of the “report pattern” areexamples, and the “report pattern” may employ other configurations.

FIG. 23 includes diagrams illustrating a modification of formatinformation to which a report pattern is attached. The formatinformation items (a) and (b) illustrated in FIG. 23 each include a“report pattern” in addition to a corresponding one of the formatinformation items illustrated in FIGS. 17(a) and 17(b). The information(flag) included in the “report pattern” is the same as that in FIG. 22,and a description of the flag information will thus be omitted. Similarto FIG. 22, each format information item illustrated in FIG. 23 may beused for reporting feedback information.

(Transmission Method of Discovery Signal)

The following illustrates a transmission method of a discovery signalcorresponding to a narrow beam transmitted by the signal transmissionunit 202 of the small base station 11.

FIG. 24 includes diagrams illustrating an example of a method fortransmitting a discovery signal corresponding to a narrow beam. In FIGS.24(a) to 24(d), a horizontal axis represents time and a vertical axisrepresents frequency. Further, “H” indicates horizontally polarizedwaves, and “V” indicates vertically polarized waves. The same shadingindicates a discovery signal corresponding to the same narrow beam(i.e., a narrow beam having the same beam ID). Further, in FIG. 24, atransmission timing of each discovery signal is not particularlyspecified; however, a discovery signal may be transmitted at apredetermined cycle, for example.

FIG. 24(a) is illustrated first. As illustrated in FIG. 24(a), thesignal transmission unit 202 transmits, at each transmission timing, adiscovery signal corresponding to the same narrow beam, for example, byfrequency hopping. The user apparatus 20 transmits the reception qualitywith respect to the discovery signal to the small base station 11 ateach transmission timing. In other words, the example of FIG. 24(a)illustrates that a transmission timing of the discovery signal (stepS301 in FIG. 13) is associated with a reporting timing of the receptionquality (step S304 in FIG. 13) on a one-to-one basis between the smallbase station 11 and the user apparatus 20.

Next, FIG. 24(b) is described. As illustrated in FIG. 24(b), the signaltransmission unit 202 transmits, at each transmission timing, adiscovery signal corresponding to the same narrow beam by performingfrequency hopping. Further, the user apparatus 20 measures the receptionquality based on the discovery signals received at multiple timings, andtransmits the measured reception quality to the small base station 11.

Next, FIG. 24(c) is described. As illustrated in FIG. 24(c), the signaltransmission unit 202 transmits, at each transmission timing, adiscovery signal corresponding to the same narrow beam via the samefrequency (subcarrier) without performing frequency hopping. Further,the user apparatus 20 measures the reception quality based on thediscovery signals received at multiple timings, and transmits themeasured reception quality to the small base station 11.

Next, FIG. 24(d) is described. As illustrated in FIG. 24(d), the signaltransmission unit 202 transmits, at each transmission timing, adiscovery signal corresponding to the same narrow beam, for example, byperforming frequency hopping with mirroring as used in PUSCHtransmission in LTE. That is, the discovery signal corresponding to thesame narrow beam is transmitted at the same frequency (subcarrier) in afixed cycle. Further, the user apparatus 20 measures the receptionquality based on the discovery signals received at multiple timings, andtransmits the measured reception quality to the small base station 11.

In accordance with the above-described methods of FIGS. 24(b) to 24(d),the reception quality may be measured from the discovery signalsreceived at multiple timings. Accordingly, it is possible to measure thereception quality with high accuracy while eliminating an adverse effectof noise included in the discovery signals. Further, in accordance withthe methods of FIGS. 24 (b) and 24(d), the frequency hopping of adiscovery signal enables a user terminal to measure the receptionquality with high accuracy while eliminating an adverse effect of thefrequency. Moreover, in accordance with the methods of FIGS. 24 (b) and24(d), since the discovery signal is transmitted with differentpolarization (vertically polarized wave and horizontally polarizedwave), the user terminal may measure the reception quality with highaccuracy.

In the present embodiment, a discovery signal corresponding to thenarrow beam may be transmitted by other methods differing from thosemethods illustrated in FIG. 24.

<Effect>

As described above, according to the embodiment, there is provided auser apparatus communicating with a base station of a radiocommunication system including the base station and the user apparatus.The user apparatus includes a first receiving unit configured to receivea plurality of first reference signals transmitted from the basestation; a detection unit configured to detect a specific antenna portvia which the plurality of first reference signals is received, or aspecific directivity pattern, among a plurality of directivity patternsgenerated by a plurality of antenna ports, in which the plurality offirst reference signals are received; a measuring unit configured tomeasure reception power of each of the plurality of first referencesignals; and a transmission unit configured to group the reception powerof each of the plurality of first reference signals into one of groupsof the specific antenna port and the specific directivity pattern viawhich the plurality of first reference signals are received to transmitthe groups of the reception power to the base station.

By the user apparatus 20, in the radio communication system having thebase station performing beam forming and the user apparatus, it isenabled to appropriately select a plurality of beams used forcommunication, from a plurality of beams formed by the base station.

In addition, the user apparatus may transmit, to the base station,header information indicating a number corresponding to the plurality offirst reference signals belonging to a corresponding one of the groups.By transmitting such header information to the small base station 11,the small base station 11 may be enabled to group the plurality of firstreference signals by direction in which the plurality of first referencesignals are received by the user apparatus 20.

The first transmission unit may group the reception power of each of theplurality of first reference signals satisfying a predeterminedcondition into the groups and transmit the groups to the base station.As a result, it may be possible to reduce a data amount for use inreporting the reception power from the user apparatus 20 to the smallbase station 11.

The user apparatus may further include a second receiving unitconfigured to receive one or more of second reference signalstransmitted from the base station; and a generating unit configured tomeasure a reception quality of the one or more second reference signalsto generate feedback information based on the reception quality. In theuser apparatus having the above configuration, the transmission unit maytransmit the reception power of each of the plurality of first referencesignals and the feedback information simultaneously to the base station.As a result, the user apparatus 20 may be enabled to collectivelytransmit the reception power of the discovery signal corresponding tothe narrow beam and the feedback information to the small base station11, and to reduce the number of signals transmitted and received betweenthe user apparatus 20 and the small base station 11.

Further, the first transmission unit may receive, from the base station,header information indicating the number of the plurality of firstreference signals belonging to the corresponding groups, and group thereception power of each of the plurality of first reference signals intothe groups based on the received header information to transmit thegroups of the reception power to the base station. As a result, thesmall base station 11 may be enabled to designate a signal format forcausing the user apparatus 20 to report the reception power of each ofthe plurality of first reference signals.

In addition, the first receiving unit may receive the plurality offrequency hopped first reference signals from the base station. Further,the measuring unit may calculate the reception power of each of theplurality of first reference signals to be transmitted to the basestation by measuring the reception power of a corresponding one of theplurality of first reference signals a plurality of times. As a result,the user apparatus 20 may be enabled to measure the reception power withhigher accuracy.

In addition, according to the embodiment, there is provided a basestation communicating with a user apparatus in a radio communicationsystem including the base station and the user apparatus. The basestation includes a first transmission unit configured to transmit aplurality of first reference signals; a first receiving unit configuredto receive, from the user apparatus, the reception power of each of theplurality of first reference signals grouped into a plurality of groups;a selection unit configured to select one of antenna ports fortransmitting one or more second reference signals based on the receptionpower of each of the plurality of first reference signals grouped intothe plurality of groups; a second transmission unit configured totransmit the one or more second reference signals from the antenna portselected by the selecting unit; a second receiving unit configured toreceive, from the user apparatus, feedback information based onreception quality of each of the one or more second reference signals;and a control unit configured to control the antenna port based on thefeedback information.

The small base station 11 having the above configuration may be enabledto appropriately select a plurality of beams used for communication,from among a plurality of beams formed by the base station, in a radiocommunication system having the base station performing beam forming andthe user apparatus.

Note that each “unit” in the configuration of each of the aboveapparatuses may be replaced with a “part”, a “circuit”, a “device”, orthe like.

Supplementary Description of Embodiment

The embodiments have been described as described above; however, thedisclosed invention is not limited to these embodiments, and a personskilled in the art would understand various variations, modifications,replacements, or the like. Specific examples of numerical values havebeen used for encouraging understanding of the present invention;however, these numeric values are merely examples and, unless otherwisenoted, any appropriate values may be used. In the above description,partitioning of items is not essential to the present invention.Provisions described in more than two items may be combined ifnecessary. Provisions described in one item may be applied to provisionsdescribed in another item (as long as they do not conflict). In afunctional block diagram, boundaries of functional units or processingunits do not necessarily correspond to physical boundaries of parts.Operations of multiple functional units may be physically performed in asingle part, or operations of a single functional unit may be physicallyperformed by multiple parts. For the sake of convenience, the userapparatus and the base station have been described by using functionalblock diagrams. These apparatuses may be implemented by hardware, bysoftware, or by combination of both. The software which is executed by aprocessor included in a user apparatus according to an embodiment andthe software which is executed by a processor included in a base stationmay be stored in a random access memory (RAM), a flash memory, aread-only memory (ROM), an EPROM, an EEPROM, a register, a hard diskdrive (HDD), a removable disk, a CD-ROM, a database, a server, or anyother appropriate recording medium. The present invention is not limitedto the above-described embodiments and modifications, and mayincorporate variations, alterations, corrections, substitutions, and thelike without departing from the spirit of the present invention.

Note that in the above embodiments, a discovery signal is an example ofa first reference signal. Further, a measurement reference signal is anexample of a second reference signal. The signal receiving unit 101 isan example of a first receiving unit and a second receiving unit. Thereception direction detecting unit 111 is an example of a detectingunit. The reception quality measuring unit 103 is an example of ameasuring unit. The signal transmission unit 102 is an example of afirst transmission unit and a second transmission unit. The formatinformation is an example of header information. The signal receivingunit 201 is an example of a first receiving unit and a second receivingunit. The signal transmission unit 202 is an example of a firsttransmission unit and a second transmission unit. The candidate beamselecting unit 203 is an example of a selection unit. The beam controlunit 204 is an example of a control unit.

The present application is based on and claims the benefit of priorityof Japanese Priority Application No. 2015-052709 filed on Mar. 16, 2015,the entire contents of which are hereby incorporated by reference.

DESCRIPTION OF REFERENCE SIGNS

-   10 macro base station-   11 small base station-   12 small base station-   20 user apparatus-   101 signal receiving unit-   102 signal transmission unit-   103 reception quality measuring unit-   104 feedback information generating unit-   111 reception direction detecting unit-   201 signal receiving unit-   202 signal transmission unit-   203 candidate beam selecting unit-   204 beam control unit-   301 RF module-   302 BB process module-   303 UE control module-   401 RF module-   402 BB process module-   403 apparatus control module-   404 communication IF

1. A user apparatus communicating with a base station in a radiocommunication system, the radio communication system including the basestation and the user apparatus, the user apparatus comprising: a firstreceiving unit configured to receive a plurality of first referencesignals transmitted from the base station; a detection unit configuredto detect a specific antenna port via which the plurality of firstreference signals is received, or a specific directivity pattern, amonga plurality of directivity patterns generated by a plurality of antennaports, in which the plurality of first reference signals are received; ameasuring unit configured to measure reception power of each of theplurality of first reference signals; and a transmission unit configuredto group the reception power of each of the plurality of first referencesignals into one of groups of the specific antenna port and the specificdirectivity pattern via which the plurality of first reference signalsare received to transmit the groups of the reception power to the basestation.
 2. The user apparatus according to claim 1, wherein thetransmission unit transmits, to the base station, header informationindicating a number corresponding to the plurality of first referencesignals belonging to a corresponding one of the groups.
 3. The userapparatus according to claim 1, wherein the transmission unit may groupthe reception power of each of the plurality of first reference signalssatisfying a predetermined condition into the groups and transmit thegroups to the base station.
 4. The user apparatus according to claim 1,further comprising: a second receiving unit configured to receive one ormore of second reference signals transmitted from the base station; anda generating unit configured to measure a reception quality of the oneor more second reference signals to generate feedback information basedon the reception quality, wherein the transmission unit transmits thereception power of each of the plurality of first reference signals andthe feedback information simultaneously to the base station.
 5. The userapparatus according to claim 1, wherein the transmission unit receives,from the base station, header information indicating the number of theplurality of first reference signals belonging to the correspondinggroups, and groups the reception power of each of the plurality of firstreference signals into the groups based on the received headerinformation to transmit the groups of the reception power to the basestation.
 6. The user apparatus according to claim 1, wherein the firstreceiving unit receives the plurality of frequency hopped firstreference signals from the base station.
 7. The user apparatus accordingto claim 1, wherein the measuring unit calculates the reception power ofeach of the plurality of first reference signals to be transmitted tothe base station by measuring the reception power of a corresponding oneof the plurality of first reference signals a plurality of times.
 8. Abase station communicating with a user apparatus in a radiocommunication system, the radio communication system including the basestation and the user apparatus, the base station comprising: a firsttransmission unit configured to transmit a plurality of first referencesignals; a first receiving unit configured to receive, from the userapparatus, the reception power of each of the plurality of firstreference signals grouped into a plurality of groups; a selection unitconfigured to select one of antenna ports for transmitting one or moresecond reference signals based on the reception power of each of theplurality of first reference signals grouped into the plurality ofgroups; a second transmission unit configured to transmit the one ormore second reference signals from the antenna port selected by theselecting unit; a second receiving unit configured to receive, from theuser apparatus, feedback information based on reception quality of eachof the one or more second reference signals; and a control unitconfigured to control the antenna port based on the feedbackinformation.
 9. A communication method executed by a user apparatuscommunicating with a base station in a radio communication system, theradio communication system including the base station and the userapparatus, the communication method comprising: receiving a plurality offirst reference signals transmitted from the base station; detecting aspecific antenna port via which the plurality of first reference signalsis received, or a specific directivity pattern, among a plurality ofdirectivity patterns generated by a plurality of antenna ports, in whichthe plurality of first reference signals are received; measuringreception power of each of the plurality of first reference signals; andgrouping the reception power of each of the plurality of first referencesignals into one of groups of the specific antenna port and the specificdirectivity pattern via which the plurality of first reference signalsare received to transmit the groups of the reception power to the basestation.
 10. A communication method executed by a base stationcommunicating with a user apparatus in a radio communication system, theradio communication system including the base station and the userapparatus, the communication method comprising: transmitting a pluralityof first reference signals; receiving, from the user apparatus, thereception power of each of the plurality of first reference signalsgrouped into a plurality of groups; selecting one of antenna ports fortransmitting one or more second reference signals based on the receptionpower of each of the plurality of first reference signals grouped intothe plurality of groups; transmitting the one or more second referencesignals from the antenna port selected in the selecting; receiving, fromthe user apparatus, feedback information based on reception quality ofeach of the one or more second reference signals; and controlling theantenna port based on the feedback information.